This invention relates generally to methods and apparatus for magnetic resonance imaging (MRI) systems, and more particularly to methods and apparatus that facilitate making and using passive shim elements that provide for low eddy currents and an improved image stability.
Achieving a high final field homogeneity in MRI magnets typically requires the use of magnetic shimming, either a fully passive or a hybrid system that includes passive shims as an integral part. Shim elements made of magnetized steel, loaded strategically on shim rails, inserted inside the bore and saturated by the main field of the magnet, compensate for manufacturing tolerances and environmental inhomogeneities. These shim elements are exposed to pulsing fields from a plurality of gradient coils which generate heat in the shim elements, raising their temperature and thus affecting magnetic field stability (both B0 and homogeneity) due to the temperature-dependent saturation BS(T) of the shims. Among major contributors to the shim heating are eddy currents induced in the conducting shims by the changing magnetic flux from the gradient coils. There is a need for efficient inexpensive passive shim elements with low eddy current heat generation.